The Old English word for "dragon" was "draca", from which we have this secondary meaning of "drake". Another common word for "dragon", probably even more common (read the Beowulf!) was "wyrm", but here semantic change has hit again even harder, because in current English the word has become "worm". Why? Keep in mind that the present day image of a dragon is somewhat different from ancient times, when dragons were conceived as reptilian, snake-like animals (think about the chinese dragons). In fact both OE words ("draca" and "wyrm") also meant "(giant) snake, serpent": in the case of "wyrm" the general idea of a crawling animal stuck during the centuries, until it modified the meaning of the word to point to a "worm".